The present invention relates to a tire, the sidewalls of which comprise a vulcanized composition based on at least one elastomer. Also provided is a process for forming an even, varnished film on at least one of said sidewalls. In particular the invention protects the tire against the effects of the migration of antioxidants and antiozonants in the sidewalls that are visible on the respective outer faces of the sidewalls.
Certain rubber compositions are very susceptible to the action of ozone, in particular, vulcanized compositions based on diene elastomers. When an article comprising such a vulcanized elastomeric composition is subjected to prolonged static and dynamic stresses in the presence of ozone, visible cracks or fissures that are more or less marked appear on the surface of the article. These cracks are oriented substantially perpendicular to the direction of the stress. Their propagation under the action of persistent stress may eventually cause complete breakage of the article.
In order to minimize the effects of ozone, antiozonants intended to reduce the formation and propagation of cracks under the above mentioned stress, have been incorporated into such articles, especially tire sidewalls composed of such vulcanized elastomers. Waxes have also been used to provide a protective coating on the surface of the sidewalls so as to ensure additional protection under static stresses. The use of these antiozonants and waxes has proved effective in minimizing cracks on the surface of the sidewalls.
Unfortunately, the most effective known antiozonants also have a very great aptitude to migrate across the elastomer substrate of the sidewall and stain and color the surfaces adjacent to said substrate. More precisely, yellowish or brown marks are observed on the surfaces of sidewalls, a phenomenon known as xe2x80x9ccolorationxe2x80x9d.
Furthermore, waxes also migrate to the surface of sidewalls, the effect which makes the surface dull and gray, or even whitish, a phenomenon known as xe2x80x9cefflorescencexe2x80x9d.
The subject of the present invention is a tire, the sidewalls of which comprise a vulcanized elastomer composition based on at least one elastomer, which makes it possible to overcome the disadvantages caused by the undesirable coloration and efflorescence phenomena.
Accordingly, the present invention provides a tire, the sidewalls of which contain a vulcanized elastomer composition, the composition comprising in combination:
a polymer containing an oxy radical of the formula xe2x80x94Oxe2x80x94 therein, the polymer having an alkyl group on one side of the oxy radical and, on the other side of the oxy radical, at least one polyalkylene oxide block of the formula (CnH2nO)z joined to a hydrogen atom at the chain end thereof, such that the hydrogen atom and the end terminal oxygen of the polyalkylene oxide block form an alcohol function, in the amount of between 0.3 and 5 phr (parts by weight per 100 parts elastomer), and
an alkali metal salt of an alkylsulphonic or alkylsulphuric acid, in an amount of between 0.5 and 10 phr.
The incorporation of the above polymer and alkali salt into the vulcanized elastomeric composition permits formation of an even, varnished film on the respective outer faces of the tire sidewalls comprising the vulcanized elastomer composition upon exposure to an atmosphere having an ozone content of at least 4 parts per hundred million, such as found in a moderately polluted urban atmosphere.
This even film has the advantage of giving said sidewalls a non-stained appearance, such that the aforementioned undesirable effects of said phenomena of coloration and efflorescence are thus completely overcome.
Furthermore, said film imparts a varnished appearance to said sidewalls, that is to say, one which shines like varnish, which significantly improves their aesthetic appearance.
According to one embodiment of the invention, the polymer having an oxy radical comprises polyoxyethylene and polyoxypropylene blocks and, for example, corresponds to the formula:
Rxe2x80x94Oxe2x80x94(C3H6O)xxe2x80x94(C2H4O)yxe2x80x94H, 
in which R is a blend of alkyl groups having from 13 to 15 carbon atoms, having a majority of 14 carbon atoms, and x varies from 3 to 15 and y varies from 3 to 5, the molecular weight of the polymer is from 500 g/mol to 1100 g/mol.
Advantageously, the polymer has a molecular weight which is about 670 g/mol (x being 5 and y being 4), or 510 g/mol (x and y each being 3), or 1050 g/mol (x being 15 and y being 4).
Preferably, the polymer having an oxy radical is present in the vulcanized elastomeric composition in an amount of about 2 phr.
According to a further embodiment of the invention, the alkali metal salt is a sodium salt of an alkylsulfonic or alkylsulfuric acid, present in said composition in an amount of about 5 phr, that corresponds to the formula:
Rxe2x80x2xe2x80x94SOzxe2x80x94Na, 
in which Rxe2x80x2 is a blend of alkyl groups having from 3 to 17 carbon atoms, and z is an integer equal to 3 or 4.
Where the salt has the formula Rxe2x80x2xe2x80x94SO3xe2x80x94Na, Rxe2x80x2 has a majority of 8 or 15 carbon atoms.
Where the salt has the formula Rxe2x80x2xe2x80x94SO4xe2x80x94Na, Rxe2x80x2 has a majority of 8 or 13 carbon atoms.
According to a further aspect of the invention, the elastomeric composition comprises at least one antiozonant and/or at least one antioxidant. The antiozonant is selected from the group consisting of 2,4,6-tris(N-1,4-dimethyl-pentyl-p-phenylene-diamino)-1,3,5-triazine and N,Nxe2x80x2-di(1,4-dimethylpentyl)-p-phenylenediamine. A suitable antioxidant is, for example, N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine.
The invention also provides a process for forming an even, varnished film on the outer face of a sidewall of a tire comprising statically exposing a tire sidewall composed of a vulcanized elastomer composition of the invention, to ambient air having an ozone concentration equal to or greater than 20 parts per 100 million for at least 48 hours, in a chamber at a temperature substantially equal to or greater than 25xc2x0 C., such that a film having a thickness of equal to or greater than 0.5 microns forms on the outer face of the sidewall. Preferably, the ambient air has an ozone content equal to or greater than 40 parts per 100 million.
This film also has the advantage of providing said sidewalls with a non-stained appearance, such that the aforementioned undesirable effects (coloration, efflorescence) are overcome, while providing a glossy appearance of the sidewall.
The aforementioned characteristics of the present invention, as well as others, will be better understood on reading the following description of several embodiments of the invention, which are given by way of illustration and not of limitation.
In the following examples, molecular weights were measured by the tonometry technique using a vapor pressure osmometer sold under the name xe2x80x9cWESCAN model 232Axe2x80x9d. For these measurements, xe2x80x9cRPxe2x80x9d toluene was used as solvent and measurements were carried out at 50xc2x0 C.